1. Field of the Invention
The present invention relates generally to integrated circuits with fuse elements and more specifically to an improved method of making fusible elements and their interconnects.
2. Description of the Prior Art
Resistive thin film links or fuses are utilized in the design of monolithic integrated circuits as elements for the storage of information in a memory array and as elements for altering the configuration of the circuitry within the integrated circuit. Memories are programmed and circuit configurations altered by "blowing" appropriate fuses using circuitry provided for that purpose.
From a circuit design viewpoint, it is highly desirable to minimize the series resistance of the fuse element between the fuse neck region and the interconnect metal. Consistent with photo lithographics capabilities and alignment tolerances, the series resistance is minimized by locating the interconnect metal as close to the fuse neck as possible.
It is also desirable to minimize the power which must be applied to the fuse in order to "blow" or program the element. The power to blow is dependent, to some degree, upon the thermal conductivity of the environment of the fuse neck region.
Prior art fuse elements illustrated in FIGS. 1 and 2 generally included a substrate 10, an insulative layer 12, interconnects 14 and 16, wherein interconnect 14 is connected to the substrate 10 through an opening 18 in the insulative layer 12, and fusible element 20 having a neck portion 22. By forming the fuse 20 directly on the insulative layer 12, a high thermal conductivity of the environment is provided and consequently, more power is needed to blow the fuse. Similarly, there is a probability of the fuse element regrowing after programming. The opposed parallel edges of interconnects 14 and 16 at the connection to fuse 16 increases the fuse resistance.
To reduce the thermal conductivity environment of the fuse, the prior art device of FIGS. 3 and 4 was developed wherein the fuse 20 is formed on top of the interconnects 14 and 16 and separated from the insulative layer 12 by an air gap 24. In addition to providing a lower thermal conductivity, the gap 24 also decreases the probability of a fuse element regrowing after programming. The opposed parallel edges of connector 14 and 16 of the prior art device of FIGS. 3 and 4 provide the same series resistance fuse element as the fuse element of FIGS. 1 and 2.
The method of making the fuse structure similar to that illustrated in FIGS. 3 and 4 is described in U.S. Pat. No. 4,032,949. This process includes four layers of metal and a plurality of selective top etching and side etching to perform the suspended fuse structure.
Thus there exists a need for a process for fabricating fuses having a low thermal conductivity, a low series resistance, and a decreased probability of fuse element regrowth after programming.